Apparatus for injecting charged particles into the magnetic field of a cyclic particle accelerator



June 13, 1967 E ETAL 3,325,713

APPARATUS FOR INJECTING CHARGED PARTICLES INTO THE MAGNETIC FIELD OF ACYCLIC PARTICLE ACCELERATOR Filed Aug. 22, 1962 v INVENTORS. M3905Sez'd? defila. 'edpa'ek Pal/e? Sun/ta United States Patent 3,325,713APPARATUS FOR INJECTING CHARGED PARTI- CLES INTO THE MAGNETIC FIELD OF ACY- CLIC PARTICLE ACCELERATOR v Milos Seidl, Zdenk Sedlacek, and PavelSunka, Prague, Czechoslovakia, assignors to Ceskoslovenskti akademie vd,Prague, Czechoslovakia Filed Aug. 22, 1962, Ser. 1 o. 218,703 Claimspriority, application Czechoslovakia, Aug. 25, 1961, 5,174/61 Claims.(Cl. 328-233) The present invention relates to cyclic accelerators ofelectrons and ions, and deals more particularly with a device forinjecting charged particles into the magnetic field of an accelerator.The invention will be explained in its application to a betatron, itbeing of course understood, that the invention is in no way limited tothis particular type of apparatus and can be used in connection with anysuitable accelerator.

It is a well known fact that the correct operation of a betatronrequires the electrons to be introduced into the magnetic field of theaccelerator with a certain initial energy and substantially in azimuthaldirection. This problem has heretofore been dealt with by two principalmethods: The older is Kersts method (US. Patent No. 2,497,891, datedFeb. 21, 1950) according to which a three-electrode gun accommodateddirectly in the accelerating space is used. The electrons, emitted bythe gun with an energy of, for instance, 60 kev., are immediatelycaptured by the guiding magnetic field. In order that an electron gun,positioned in this way, should hender the circulating electrons to theleast extent, its dimensions must be kept at a minimum; this necessity,however, places a limit on the injection voltage. The number of capturedelectrons increasing with the injection energy, it is preferable to useas high an injection voltage as possible.

The aforementioned drawback is removed by the injecting method accordingto Gund (German Patent No. 810,286, dated Aug. 9, 1951, and No. 891,433,dated Sept. 28, 1953). According to said method an electro-staticdeflector is used, by means of which the electrons are injected inazimuthal direction to the guiding magnetic field from an electron gun,which can be accommodated outside the actual accelerating region. Thisarrangement does not require a compact electron gun, and the injectionenergy of the electrons may be substantially increased.

The use of an electrostatic deflector entails, however, a number ofgrave disadvantages:

(1) The focussing of the electron beam emerging from the electron gun isimpaired by electron-optical deficiencies of the deflector, which arethe greater the larger the angle through which the particles aredeflected, but which are present even if the deflector does not bend thebeam, but merely overcomes the forces of the magnetic field.

(2) The trajectory of the electrons within the deflector issubstantially influenced by the arrangement of the magnetic field of theaccelerator, which can difler in products of various origin. If theaperture of the deflector is not sufliciently large, injected particlesmay be lost on the deflector electrodes.

(3) An electrostatic deflector requires a separate high voltage source.

(4) The angle under which the charged particles are injected into theguiding magnetic field of the accelerator, is constant during theinjection period (lasting several microseconds) only if the voltageacross the gun and across the deflector varies with time in the samemanner, which is diflicult to achieve. A variation in the injectionangle with time results in a reduction in the magnitude of the capturedcharge.

The aforesaid difliculties are avoided according to the invention by theprovision of a channel that is shielded from the magnetic field andtherefore free from deflecting magnetic forces, said channel serving forthe admission of electrons to the magnetic field.

The novel feature of the invention, including various novel details ofconstruction and combination of parts, will now be more particularlydescribed with reference to the accompanying drawings and thereafterpointed out in the claims.

In the drawings:

FIG. 1 is a diagrammatic representation showing the vacuum chamber andinjection device of a betatron,

FIG. 1a shows the injector of FIG. 1 and the windings connected thereto,

FIG. 2 shows the injector tube of the apparatus of FIG. 1 in aperspective view and partly in transverse section, and

FIG. 3 shows a modified compensating arrangement for the betatron ofFIG. 1 in a diammat-ric view.

Referring to FIG. 1, there is seen an electron gun 3 mounted in atangential tubular extension of a vacuum chamber 2. The vacuum chamberis placed in the usual way in the magnetic field of a betatron. Theelectron gun is positioned outside the poles of the betatron magnet, andcan therefore easily be designed for the required resistance againsthigh voltage and for correct focussing of the beam. The gun produces acylindrical electron beam with an energy of for instance kev., which isfocussed so as to make the beam divergence in the plane A at the end ofan injector tube 4 equal to zero. The injector tube 4 is coaxiallyaligned with the electron gun 3. It is connected to the semi-conductivecoating of the vacuum chamber 2 and grounded thereby. The gun, as wellas the injector tube, are made of a ferromagnetic material of highpermeability, which shields the electron beam 5 from the externalguiding magnetic field. In the cylindrical injector tube, which has adiameter D and a wall thickness d (FIG. 2) and is made of a material ofa permeability ,u, the magnetic field is d (A -U 1 D I times smallerthan the magnetic field outside the injector, tube. The electron beam 5passes from the cathode 6 in the gun 3 to the plane A through anegligibly small magnetic field, independent of the external guidingfield. Having passed the plane A, the electron beam enters the guidingmagnetic field of the betatron tangentially to an orbital circle havinga centre S and passing through the intersection M of the injector axiswith the plane A. Small corrections of the injector angle may berequired to correct errors in the mounting of the vacuum chamber in themagnet, and can be effected by a system of deflection plates similar tothose'usedin a cathode ray tube.

FIG. 1 shows deflection plates 7 for deflecting the'beam in a horizontalplane and plates 8 for deflection a 3 vertical plane. The diameter D ofthe cylindrical injector tube 4 is preferably just slightly larger thanthe diameter of the electron beam and, with a correctly focussed beam,it may amount to 5-10 mm.

The device of the invention provides a channel in the magnetic field ofthe betatron which is substantially free from the disadvantages of anelectrostatic deflector, but it causes a deformation of the externalmagnetic field.

If the magnetic field of the betatron is not entirely homogeneous andconstant the injector tube is subjected to forces which may exceed themechanical strength of the injector or of the electron gun.

The last-mentioned difiiculties are avoided by an additional feature ofthe present invention. As the injection of electrons into the betatrontakes but a very short fraction of the accelerating cycle (about 1.5l'0* of the cycle), we make the injector tube from a magnetically softmaterial whose hysteresis loop is of substantially rectangular shape. Weselect the wall thickness of the injector tube in such a manner that atthe moment when the injection is terminated, the magnetic induction inthe injector wall is equal to the saturation limit :13, of the injectormaterial. When injection is terminated, the injector tube materialimmediately becomes magnetically saturated and, after a'certain time,its permeability drops to nearly 1, whereby the injector does not causea deformation of the external magnetic field, and the force action onthe injector drops almost to zero. If the magnetic induction of theguiding field equals B then the wall thickness of the injector tube mustbe When the wall thickness is chosen according to the preceedingformula, and the permeability of the material amounts to the magneticfield within the injector does not exceed 1% of the external guidingfield, so that the path of electrons within the injector is, to allpractical purposes, not influenced by the external field. When strongershielding from the external magnetic field is i: I sin (,0

The angle (,0 is shown in FIG. 2. It is in a radial plane with respectto the common axis of the tube 4 and the beam 5, and has its apex in theaxis. A sinusoidal current density distribution can be effected, forexample, by means of a winding 9 shown in FIGS. 1 and 2.

'The current feeding the compensation winding can be derived from thecurrent which energizes the winding 110 of the guiding magnet 11 bymeans of a saturable transformer. The transformer core 12 is preferablymade from the same material as the injector tube 4, and the winding ofthe transformer 12 is designed so as to make the saturation in thetransformer core equal to the indution B of the injector tube materialat the magnet when injection is terminated. This ensures a variation ofthe compensating current in precise synchronization with variations inthe intensity of the magnetic field. The primary winding of thetransformer 12 is connected in series to the magnet winding 110 whichensures the correct maximum value of the compensating currentindependent of fluctuations to which the power supply of the betatronmagnet 11 may be subjected.

The deformation of the guiding magnetic field caused by the injectortube 4 maybe compensated, alternatively,

4 as shown in FIG. 3 by a set of strips 10 made of a magnetically softferromagnetic material and placed above and below the injector tube 4between the poles of the betatron magnet 11.

By a suitable arrangement of the strips 10 it is possible to obtain in aplane B spaced from the strips by a distance equal to the gap betweenthe strips, substantially the same distribution of magnetic potential asin a field not disturbed by the injector tube. The greater the number ofstrips, the more accurately the deformation of the filed can becompensated. In practice, however, two or three strips are sufiicient.

The forces acting on the injector tube are very small because thematerial of the injector becomes saturated soon after the injection. Forinstance, in a betatron of 15 mev., the force will exceed not 10 to 20grams per centimeter of injector tube length. If the injector isproperly designed, such a force is unable to influence its operation orto overcome its mechanical strength.

What we claim is:

1. In a cyclic particle accelerator, in combination:

(a) electromagnetic means for generating a magnetic guiding field,

( 1) said field defining an arcuate orbital path for electricallycharged particles;

(b) a source of a beam of electrically charged accelerated particles,

(I) said source being outside said field, and

(2) said beam having an axis substantially tangential to said path;

(c) a tubular injector member of magnetically soft ferromagneticmaterial elongated in the direction of said axis,

(1) a longitudinal portion of said member being in said field,

(2) said member enveloping a portion of said beam extending from aregion adjacent said source to a portion of said field adjacent saidpath,

(3) said source being adapted to generate said beam in an injectionpulse of limited duration, and said injector member being dimensioned toreach magnetic saturation in said field in a time not substantiallyexceeding said period,

(4) said portion of said injector member being adapted to deform saidfiled during the duration of said pulse; and v (d) compensating meansfor compensating the deforming effect of said injector member on saidfield.

2. In an accelerator as set forth-in claim 1, said compensating meansincluding means for passing an electric cur-rent axially over saidinjector member, the density of said current being sinusoidallydistributed about said axis, and means for varying the magnitude of saidcurrent in synchronization with variations of the intensity of saidfield.

3. In an accelerator as set forth in claim 2, said electromagnetic meansincluding an energizable accelerator magnet having a winding, saidcurrent passing means including a saturable transformer, and acompensating conductor means on said injector member, said transformerhaving a primary winding in series circuit with the winding of saidaccelerator magnet, a secondary winding in circuit with said conductormeans, and a core of magnetically soft ferromagnetic materialdimensioned to reach magnetic saturation in a time not substantiallyexceeding said period when said magnet is being energized.

4. In an accelerator as set forth in claim 1, said electromagnetic meansincluding a magnet having two poles, said portion of said injectormember being interposed between said poles, and said compensating meansincluding .a plurality of strips of ferromagnetic material inter- Posedbetween said injector member and 'said'poles, said strips beingdimensioned to reach magnetic saturation in said field in a time notsubstantially exceeded said period.

5. In an accelerator as set forth in claim 1, a chamber adapted to beevacuated, said chamber having an arcuate Wall and a tubular extensionprojecting substantially tangentially outward of said chamber from saidwall, said electromagnetic means generating said field in said chamberand defining said path in said chamber, said source being mounted insaid extension, and said injector member extending from said tubularextension into said chamber, said longitudinal portion of the injectormember being in said chamber.

References Cited UNITED STATES PATENTS 2,497,891 2/1950 Kerst 313-62 52,721,949 10/1955 Gund et a1. 3l362 2,812,463 11/1957 Teng et a1. 313622,830,211 4/1958 Kaiser et a1 31362 HERMAN KARL SAALBACH, PrimaryExaminer.

10 DAVID J. GALVIN, Examiner.

S. CHATMON, JR., Assistant Examiner.

1. IN A CYCLIC PARTICLE ACCELERATOR, IN COMBINATION: (A) ELECTROMAGNETIC MEANS FOR GENERATING A MAGNETIC GUIDING FIELD, (1) SAID FIELD DEFINING AN ARCUATE ORBITAL PATH FOR ELECTRICALLY CHARGED PARTICLES; (B) A SOURCE OF A BEAM OF ELECTRICALLY CHARGED ACCELERATED PARTICLES, (1) SAID SOURCE BEING OUTSIDE SAID FIELD, AND (2) SAID BEAM HAVING AN AXIS SUBSTANTIALLY TANGENTIAL TO SAID PATH; (C) A TUBULAR INJECTOR MEMBER OF MAGNETICALLY SOFT FERROMAGNETIC MATERIAL ELONGATED IN THE DIRECTION OF SAID AXIS, (1) A LONGITUDINAL PORTION OF SAID MEMBER BEING IN SAID FIELD, (2) SAID MEMBER ENVELOPING A PORTION OF SAID BEAM EXTENDING FROM A REGION ADJACENT SAID SOURCE TO A PORTION OF SAID FIELD ADJACENT SAID PATH, (3) SAID SOURCE BEING ADAPTED TO GENERATE SAID BEAM IN AN INJECTION PULSE OF LIMITED DURATION, AND SAID INJECTION MEMBER BEING DIMENSIONED TO REACH MAGNETIC SATURATION IN SAID FIELD IN A TIME NOT SUBSTANTIALLY EXCEEDING SAID PERIOD, (4) SAID PORTION OF SAID INJECTOR MEMBER BEING ADAPTED TO DEFORM SAID FILED DURING THE DURATION OF SAID PULSE; AND (D) COMPENSATING MEANS FOR COMPENSATING THE DEFORMING EFFECT OF SAID INJECTOR MEMBER ON SAID FIELD. 